This invention relates generally to wind turbines, and more specifically to methods and apparatus for controlling operation of a wind turbine.
Generally, a wind turbine includes a rotor having multiple blades. The rotor is sometimes mounted within a housing, or nacelle, that is positioned on top of a base, for example a truss or tubular tower. At least some known electric utility grade wind turbines (i.e., wind turbines designed to provide electrical power to an electric utility grid) can have rotors of 30 meters (m) (98 feet (ft)) or more in diameter. The rotor blades transform mechanical wind energy into a mechanical rotational torque that drives one or more generators. The generators are sometimes, but not always, rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the turbine rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into the electric utility grid. Gearless direct drive wind turbine generators also exist.
Seasonal changes to the ambient air conditions, for example changes to ambient air temperature and/or pressure, may affect performance of at least some known wind turbines. For example, the normal international engineering code (IEC) design envelope of a wind turbine defines loads acting on the wind energy turbine within a temperature range from about +40 degrees Celsius (° C.) (about 100 degrees Fahrenheit (° F.)) to about −20° C. (about −30° F.). Operation of a wind turbine below this temperature range may require new load calculations which will exceed the design load envelope if no countermeasures are taken, possibly resulting in the need of new, reinforced components. At least some known wind turbines, when subjected to cold weather conditions with ambient air temperature values below the lower temperature limit of the allowable temperature range, are shut off, which is disadvantageous insofar as no electric output power is generated.
Another example of seasonal changes affecting wind turbine performance is that air temperature-corrected turbine performance of at least some known wind turbines may be lower in the summer than in the winter. For example, a probability of the rotor blades of at least some known wind turbines to stall increases during summer conditions when ambient air temperatures are typically higher. Such stalling reduces a potential electric power output of the wind turbine. Moreover, reestablishment of airflow around at least some known wind turbine rotor blades after stalling may cause a short-term increase in generator speed and/or electric power output that may be difficult for a controller of the wind turbine to process. Such controller processing difficulty may increase a probability of the wind turbine to be disconnected from an electric grid due to over-speed and/or over-production conditions.